1. Field of the Invention
The invention relates to an image reader such as a copying machine, a facsimile, and a scanner, which reads images formed on a document that is an object to be read, more specifically, an image reader, which reads images on a sheet document while moving the document by an automatic document feeder.
2. Description of the Related Art
As image readers, a type which reads images on a document while moving a reading optical system upon placing the document on a platen glass, and a type which reads images on a document while moving the sheet document by an automatic document feeder upon fixing a reading optical system to a document reading position are generally known. Comparing these systems, the latter image reader, which moves a sheet document, is more advantageous than the former one, which moves the reading optical system, in terms of improvement in document image reading speed.
However, in the case of the latter image reader, if dust adhering to a document contaminates a contact glass at the document reading position or adheres to the contact glass, such contamination or dust is always read since the reading optical system is fixed at the document reading position. This causes a stripe noise in the image reading results.
In order to eliminate such a problem particular to an image reader using a document automatic feeding system, various techniques have been conventionally proposed. For example, a technique (for example, JP-A-9-139844) exists in which a plurality of photoelectric transducers are arranged in a document feeding direction, the reading results by the photoelectric transducers with respect to the same position on a document are compared, and when a difference exists between the reading results, the difference is detected as a noise component, and the noise component is eliminated. Also, a technique (JP-A-2000-152008) exists in which when a noise component is detected, the noise component is eliminated by using the reading result of one of the photoelectric transducers.
However, both the abovementioned techniques according to the related art corresponds to monochrome image reading. If the techniques are applied to color image reading, it is necessary to construct an image reader so that at least two sets of three photoelectric transducers having spectral sensitivity characteristics of red (R), green (G), and blue (B). That is, it is necessary to provide a total of six photoelectric transducers or more in the document feeding direction. Therefore, power consumption by the photoelectric transducers increases, and the heat release value from these elements also increases.
Also, it is necessary that in accordance with the number of photoelectric transducers, it is necessary to provide at least six image processing circuits, which apply analog processing and shading correction to output signals from the photoelectric transducers. Therefore, the circuit scale and power consumption increase, resulting in a remarkable increase in cost.